1. Field of the Invention
The present invention relates to a carrier for electrophotography and a two-component developer.
2. Discussion of the Related Art
As image forming apparatuses such as electrophotographic copiers, two-component image developers using a two-component developer including magnetic carrier for developing electrostatic latent images and one-component image developers using only a toner are known.
Typically, the two-component image developers include a magnet roller formed of a magnet including plural magnetic poles and a developing sleeve as a rotatably-supported cylindrical developer bearer. On the surface of the developing sleeve, a magnetic brush is formed of the two-component developer in which a toner adhered to a carrier. The magnetic brush is transferred to a developing area of an image bearer to develop. The toner is stirred and mixed with the carrier in the two-component image developers to be charged, and toner has stable chargeability and stably produces good images.
The carrier includes a core material coated with a resin materials in many cases for the purpose of preventing a toner from being spent on the surface of a carrier, forming a uniform surface of the carrier, preventing the surface from being oxidize, preventing deterioration of moisture sensitivity of the carrier, extending the life of a developer, preventing the carrier from adhering to the surface of a photoreceptor, protecting a photoreceptor from being scratched or abraded with the carrier, controlling the charge polarity or adjusting charge quantity, etc. As a carrier having a core material coated with a resin, Japanese published unexamined application No. 58-108548 discloses a carrier having a core material coated with a specific resin material; Japanese published unexamined applications Nos. 54-155048, 57-40267, 58-108549, 59-166968 and 6-202381, and Japanese published examined applications Nos. 1-19584 and 3-628 disclose carriers having core materials coated with resins including various additives; Japanese Patent No. 3120460 discloses a carrier, the surface of which an additive adheres to; Japanese published unexamined application No. 9-160304 discloses a carrier whose resin-coated core material has a coated layer including an electroconductive particulate material larger than the thickness of the layer; etc.
Japanese published unexamined application No. 8-6307 discloses a carrier having a core material coated with a resin including a benzoguanamine-n-butylalcohol-formaldehyde copolymer as a main component. Japanese Patent No. 2683624 discloses a carrier having a core material is coated with a cross-linked material of a melamine resin with an acrylic resin.
However, these carriers have insufficient durability and are insufficiently prevented to adhere to a photoreceptor. Particularly, the toner spent on the surface of a carrier destabilizes charge quantity and the abrasion of the coated layer deteriorates the resistivity of a carrier. Quality images can initially be produced, but the more produced, the lower the image quality. Therefore, the carriers need improvement.
In order to solve such problems, Japanese published unexamined application No. 2006-058811 discloses a carrier having a double-layered core material formed of an acrylic resin layer having high adhesiveness thereto and a silicone resin layer having a low surface energy and covering the acrylic resin layer.
Japanese Patents Nos. 3808120 and 3973313 disclose using an acrylic-modified silicone resins having respective good properties of an acrylic resin and a silicone resin to improve chargeability of the resultant carrier, adhesiveness to a core material thereof and resistance thereof to the toner spent.
The carriers disclosed in Japanese Patents Nos. 3808120 and 3973313 improve adhesiveness of the coated resin to a core material to prevent deterioration of image quality due to abraded layer and toner spent. However, these carriers are likely to have deteriorated coated layer due to a stress when a developer including the carrier is stirred, such as abraded layer due to insufficient mechanical strength, and have room for improvement.
Because of these reasons, a need exists for a carrier in which adhesiveness of a coated resin to a core material thereof is improved, preventing deterioration of image quality due to toner spent and having improved durability.